If you are going for it, do lots and lots of research
Write down a checklist of all the shots you'll need - the day will be very fast paced and you'll easily forget something otherwise.
The venue for the ceremony - do they allow flash photography inside? If so, I still would not blat away with a flash during the vows and service, but that could help you out for other shots before and after the ceremony. The D90 is a great camera, but you'll be pushing it at ISO1600 so get good results, so the faster the lenses you have, the better.
A 24-70 won't be wide enough (even though I used one on my D90 for 1 1/2 years, it's just not wide enough for group shots, shots of the service etc. At the long end it's fine, but otherwise not), which is why I recommended the 17-55. Yes, it's a boring focal range for an expensive zoom, but it would cover you as a single lens for just about everything. 3rd party manufacturers have f2.8 zooms in that range as well, of you could look into hiring a lens (in which case hire it for the week to get used to it)
In terms of camera settings, when you're not using flash I'd look into setting your auto ISO to a maximum of whatever you'll comfortable on the D90 (I'd say 1250 to 1600) and a minimum shutter speed of 1/80th if you're using a focal length up to 55mm. Then you don't have to worry about exposure so much, and the camera will help you through.
Research everything though - order for group shots after the service, schedule for the day, recce the locations, see where you'll be able to shoot and find the light. Find some nice spots for some bride and groom shots between the service and reception if possible. What are the bride and groom expecting in terms of style? Will you have access to the bride getting prepared, and the groom and best man/ushers getting ready? Get it all mapped out.
A good site to look at for some inspiration is www.fredmiranda.com
. In the forum area there's a whole section on Wedding Photography.